An image intensifier tube includes three main components, namely: a photocathode, a phosphor screen (anode), and a microchannel plate (MCP). The MCP is positioned intermediate to the photocathode or anode. The components are usually housed in a tube. As is well known, the photocathode is extremely sensitive to low radiation levels of infrared light in the 580-900 nm (red) spectral range. The MCP is a thin glass plate having an array of microscopic holes through it. Each hole is capable of acting as a channel-type secondary emission electron multiplier. When the microchannel plate is placed in the plane of an electron image in an intensifier tube, one can achieve a gain of up to several thousand or greater. Since each channel in a microchannel plate operates nearly independently of all the others, a bright point source of light will saturate a few channels, but will not spread out over adjacent areas. This characteristic of "local saturation" makes these tubes more immune to blooming at bright areas. It is sufficient to say that the microchannel plate is an extremely important component of an image intensifier and can also be used in conjunction with other devices as photo tubes and so on. Such microchannel plates have been used in image intensifiers for many years and such uses date back to the 1970's.
Techniques used in the manufacture of MCPs are similar to those used for fiber optic plates. Coaxial glass rods, with a soluble etchable glass core, surrounded by an insoluble glass cladding, were drawn down to the required diameter, usually in two stages. The multiple drawn fibers are then fused together and the total bundle sliced into wafer plates and then polished. To finish the channel plates, the core glass is etched out and the remaining channel glass is reduced to form a semiconducting surface on the channel walls. Finally, the electrodes to the plate are added by evaporating nichrome or some other material over the polished faces of the plate. Early microchannels plates were produced by such techniques, and in the 1970's they had thicknesses of about 0.5 mm and a channel diameter of 12 micrometers.
The use of microchannel plates in image intensifiers, as indicated, is widely known and for an example of such an image intensifier with a microchannel plate reference is made to U.S. Pat. No. 5,023,511 entitled OPTICAL ELEMENT OUTPUT FOR AN IMAGE INTENSIFIER DEVICE issued on Jun. 11, 1991 to E. Phillips and assigned to ITT Corporation, a predecessor of the assignee herein.
Basically, MCPs are two dimensional arrays of electron multipliers. An incoming electron enters the input of the MCP striking the channel wall. With voltage applied across the MCP the primary electrons are amplified, generating secondary electrons. The secondary electrons exit the back end of the MCP and diverge or spread out. This divergence increases the spot size of the image spot, and decreases the device resolution. Thus, decreasing the center-to-center spacing and the channel diameter of the MCP, operates to decrease the spread of the electrons. This then operates to increase the resolution of the MCP and therefore, the device the MCP is operating in. As indicated, such devices are image intensifiers, but other devices can be employed as well.
As indicated, the fabrication of MCPs is a fiber drawing process. An etchable core is drawn down with a surrounding lead silicate tube. The single fibers are bundled and redrawn into hexagonal multifibers. The multifibers are packed into a glass tube and then fused together into a solid boule of glass. The boule is sliced and polished into plates. The plates are etched and the core rods are removed leaving the channels. The channels are then activated and metallized. Modern production MCPs, fabricated with a two draw process, have center to center sizes down to 8 microns, with limited production at 6 micron. Smaller and smaller center to center spaced MCPs are needed to improve resolution and MTF in current image intensifiers and devices that use MCPs in imaging applications. The smaller center to center spacing results in smaller channels and the smaller channel focuses the exiting electrons and decreases the size of the imaging spot.
Thus, based on prior art techniques, which are briefly described above, in order to decrease the center to center spacing and the channel diameter, glass fibers must be drawn, stacked, redrawn and stacked again. Current production fiber sizes are very small (0.015") and are difficult to work with. The number of fibers per unit area increases by the inverse square of the center to center spacing, and is currently at 12 million channels per square inch. The small fiber size compounded with the number of fibers, makes fabricating these high resolution, small pore MCPs extremely difficult if not totally impractical.
As the channel decreases in size, the channel wall also decreases. Thus, there is less glass to fill the voids between the fibers. As the glass flows into these voids during the draw and fusion processes, the distance between both the channels and the channel itself will become distorted. In the worst case, several channels will flow together to become one large channel. This leads to non-uniform gain and selective emission points. Thin channels walls will typically break, forming particles that act as electron concentrators and operate to cause spurious emissions.
It is an object of the present invention to produce a fiber architecture that can produce micron and submicron MCPs with high channel density and uniform channel geometry. These MCPs, as indicated, will have improved MTF, resolution and signal-to-noise ratios. The technique to be described includes a method of fabrication to produce micron and submicron MCPs by using a three draw process.